I already forgot about those nasty marks on the bonnet since I fitted non-return valves last year. During that time I had 3 or 4 single marks (nothing like before) and they where gone when I washed the car, no problem at all, it did not require special attention / treatment like before.

FOCA, sorry mate, but it looks like you did not read the whole thread and linked thread (post #4). Do it first please. It works, for me and many other Focus users from different countries. I found this solution on other forum and shared here.